Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 25, 2015
Valid: June 26-30, 2015 (Friday – Tuesday)

Summary:

Unsettled weather through the period will result in generally Good air quality. Precipitation will be a factor through Sunday with the heaviest rainfall expected Saturday night into Sunday. There will be a brief period of clearing Monday but not significant enough to lead to any major increases in pollutant levels.

Discussion:

The weather models are in close agreement on the synoptic features throughout the medium range period. The 06Z GFS and 00Z ECMWF were consulted for this analysis. The 06Z NAM deviated noticeably from the global models by 12Z Friday, making it an outlier. The long-anticipated upper level pattern shift will begin to take shape tomorrow as an upper level short wave over IL in the base of a shallow trough strengthens and begins to dig. It is obvious that the GFS is providing a stronger solution compared to the EC, although the surface impacts of this discrepancy are slight with respect to air quality in the Mid-Atlantic. The stationary front along which the resulting surface wave is traveling will be oscillating between the Mason-Dixon Line and northern VA, bringing clouds and showers to the Mid-Atlantic throughout the day Friday. The larger short wave aloft will bring the first drops of rain to the southern and central Mid-Atlantic (SMA, CMA) between 00Z and 06Z Saturday; the first signs of a Saturday washout for the entire region. Around 06Z Saturday the trough will dig far enough south for the base to reach the Gulf Coast, placing a strong area of short wave energy over the northern Ohio River Valley (ORV). By this time the GFS short wave is significantly stronger than the EC’s, but otherwise their trough characteristics are virtually identical. Both solutions develop a small vort max over the Delmarva around 00Z Sunday, which the surface analyses show will increase rainfall and possibly bring convective activity to the Bay Area on Saturday evening. By 12Z Sunday the center of the surface low will be in PA, with the associated cold front sprawled along the eastern seaboard. This front placement will continue the threat for precipitation along the coast while high pressure builds over inland areas across the CMA and SMA. By this point the EC has caught up to the GFS in terms of shortwave strength, placing a winter storm–like closed area of energy aloft over western PA. Upper level analyses for Monday morning lift the base of the trough to the Tennessee River Valley, with our first area of energy moving through northern New England and a second shortwave moving in from the Northern Plains across the Great Lakes region. This energy will bring another substantial surface wave through the Mid-Atlantic, making Monday’s high pressure relatively weak and short-lived. Despite model differences regarding the exact strength and timing of this second wave, there is good agreement when it comes to the resulting surface impacts: a warm front will be pushed northeastward through the entire region on Tuesday morning shortly before a cold front follows in its path, both of which will bring widespread clouds and precipitation to the entire Mid-Atlantic.

Friday will be another day of generally Good air quality as a stalled frontal boundary will oscillate between the Mason-Dixon Line and NC. Localized heavy precipitation in the CMA Thursday night into Friday morning will clean out the atmosphere for Friday. This frontal boundary will be the focus for clouds and precipitation. The 03Z SREF and 00Z NAM 4km models show most of the convection in southern VA and NC Friday afternoon, and most of the overcast skies north of the VA/NC border. This suggests that chances for substantial ozone or PM2.5 production will be limited. There may be a few isolated locations in NC along the stalled frontal boundary, where clouds and convection are less prominent, that may see Moderate ozone. Lingering Saharan dust and smoke from agricultural fires may also push PM2.5 into the Moderate range, especially along the stalled front.

A wave of low pressure that formed along the stalled frontal boundary in the TRV on Friday will move northeastward into the ORV and impact the region on Saturday. The heaviest rain will likely be in the NMA in association with the closed low at 500 mb. Breezy winds, overcast skies, and heavy rain will clean out the atmosphere for the entire region, keeping air quality well within the Good range.

Precipitation will continue Saturday night into Sunday morning for most of the region as this wave of low pressure continues to move northeastward along the US/CAN border. By 12Z Sunday, the wave will be in the vicinity of the PA/NJ border. As a result clouds rain showers will continue for most of the day in the NMA. A cold front associated with the wave of low pressure will move through the CMA and SMA in the afternoon, promoting clear skies. Despite this late afternoon clearing, breezy winds coupled with the Saturday’s washout should ensure Good air quality for the entire region on Sunday.

A weak area of high pressure will build into the area on Monday, promoting clear skies throughout most of the region. Good air quality is as the precipitation from Saturday and Sunday coupled with the passage of a cold front on Sunday cleaned out the atmosphere. A few locations may see Moderate ozone on Monday, especially in the SMA under the center of high pressure. Locations along the I-95 corridor may also see Moderate ozone.

Another surface low pressure system will develop on Tuesday in the Midwest, bringing more widespread clouds and precipitation to the region. Air quality should stay in Good range for most of the region.

-DeBoe/Eherts/Ryan